1. Field of the Invention
The present invention relates to automobile exhaust gas catalytic converter and more particularly to an automobile exhaust gas catalytic converter that has a specific structure of a catalyst layer that includes Rh as a precious metal and the other catalyst layer that includes Rh and Pt or Pd as precious metals to be capable of providing a high NOx reduction performance under a condition where an air-fuel ratio fluctuates.
2. Description of Related Art
The exhaust gases that are discharged from automobile includes HC, CO, and NOx, and these substances are emitted into the air after converted into non-toxic substances by an exhaust gas catalytic converter. As typical exhaust gas catalytic converter used herein, three-way catalytic converter is widely used in which the precious metals such as platinum (Pt), rhodium (Rh), and palladium (Pd) are supported on porous oxide supports such as alumina (Al2O3), silica (SiO2), zirconia (ZrO2), and titania (TiO2).
A three-way catalytic converter oxidizes HC and CO and reduces NOx in the exhaust gases that are discharged to the downstream side of an engine to convert them into non-toxic substances. The three-way catalytic converter is most effective when the catalytic converter receives the exhaust gas in a stoichiometric atmosphere that is burnt in near a theoretical air-fuel ratio. However, improvement of fuel efficiency is highly demanded in recent years, and the exhaust gas catalytic converter is subjected to rapid atmospheric fluctuations such that the number of fuel-cut is increased at high temperature of 1,000° C. in some cases, or the oxidation or the reduction is performed at high temperature based on the fluctuation of the air-fuel ratio. Such the rapid atmospheric fluctuations significantly accelerate catalytic deactivation in which grain growth of precious metal component occurs in the three-way catalytic converter, and thus the performance of the catalytic converter deteriorates.
Among the components of the three-way catalytic converter, Rh is an essential component, and ZrO2 and Al2O3 are used as a support in order to prevent the grain growth of Rh. In addition, the technique for improving heat resistance of the supports is known. However, an attempt to prevent the grain growth of Rh by improving the heat resistance of the supports exhibits an effect on thermal deterioration but has insufficient effects on the prevention of Rh deterioration due to the atmospheric fluctuation. On the other hand, an attempt to change the structure of the catalyst layer that is formed on the base material is made in order to improve a catalytic performance of the exhaust gas catalytic converter.
For example, Japanese Patent Application Publication No. 2006-326428 (JP-A-2006-326428) describes that an exhaust gas catalytic converter, which has an A-region of a catalyst coating layer on an upstream side which contains Rh only or Rh and a precious metal other than Rh in the proportion of 1:1 by weight and a B-region of the catalyst coating layer on a downstream side which has higher proportion of the precious metal other than Rh, has high NOx reduction performance at high temperature. In addition, JP-A-2006-326428 describes HC 50% oxidation rate achievement temperature as a specific example. However, the NOx reduction performance after the catalytic converter is subjected to an endurance test under an air-fuel ratio fluctuating condition is not mentioned in the description.
Published Japanese Translation of PCT application No. 2007-38072 (JP-A-2007-38072) describes an exhaust gas catalytic converter that has a catalyst base material and a catalyst coating layer formed on the catalyst base material, in which the catalyst coating layer has an upstream section that is provided on the upstream side in an exhaust gas flow direction and a downstream section that is provided on the downstream side, and the upstream section has a layer structure that includes an upstream section inner layer and an upstream section outer layer. JP-A-2007-38072 also describes the exhaust gas catalytic converter that uses the combination of Rh and the precious metal other than Rh in the downstream section and that includes only Rh as the precious metal in both the upstream section and the downstream section. However, the NOx reduction performance after the catalytic converter is subjected to an endurance test under an air-fuel ratio fluctuating condition is not mentioned in the description.
Furthermore, Japanese Patent Application Publication No. 2010-5592 (JP-A-2010-5592) describes an exhaust gas catalytic converter that has an upstream catalyst layer which is thin and contains Pd and a downstream catalyst layer which includes an inner catalyst layer containing Pt on a downstream side and an outer catalyst layer containing Rh, in which the ratio of the thickness of the outer catalyst layer with respect to the thickness of the inner catalyst layer is 1.5 through 2.5. JP-A-2010-5592 also describes specific examples such that Pd and Pt are independently used as the precious metal other than Rh. However, the NOx reduction performance after the catalytic converter is subjected to an endurance test under an air-fuel ratio fluctuating condition is not mentioned in the description.
That is, the exhaust gas catalytic converters that have been disclosed in the aforementioned JP-A-2006-326428, JP-A-2007-38072, and JP-A-2010-5592 deliver insufficient NOx reduction performance after the catalytic converters are subjected to the endurance test under the air-fuel ratio fluctuating condition. Therefore, an automobile exhaust gas catalytic converter that has higher NOx reduction performance than related arts is demanded.